The Oklahoma Department of Environmental Quality (DEQ) aims to make certain U.S. Environmental Protection Agency (EPA) water quality guidance state law. DEQ’s proposed changes would require the use of the copper biotic ligand model to develop site-specific copper criteria, as well as the use of fish tissue studies for selenium site-specific criteria.
EPA water quality guidance under the Clean Water Act is not legally binding, but provides recommendations that states can follow if desired. DEQ’s proposals, however, would become enshrined in state law if approved by the Oklahoma Water Quality Management Advisory Council, Environmental Quality Board, and the Oklahoma Legislature. This Delve explains the impacts of these proposed changes and their approval process.
The Copper Biotic Ligand Model
The biotic ligand model (BLM) is a predictive tool for developing site-specific water quality guidelines for copper. It can account for changes in metal toxicity based on local water chemistry information. EPA’s 2007 aquatic life freshwater quality criteria for copper are based on this model. These criteria are recommendations and not legally required for states and Tribes to implement.
DEQ, however, is proposing to add language to Oklahoma Administrative Code (OAC) 252:730 (Oklahoma’s Water Quality Standards) requiring the use of the copper BLM to develop site-specific copper criteria. This proposal would also remove the option for using the Water Effects Ratio (WER) for site-specific copper criteria, as well as site-specific metals criteria that weren’t EPA-approved.
The existing site-specific criteria developed for various organizations discharging into Oklahoma receiving waters and cited in Appendix E of OAC 252:730 were developed using the WER methodology. It is not clear whether those copper limits will need recalculation based on the BLM methodology.
Fish Tissue Monitoring and Selenium Aquatic Life Criterion
Selenium is a naturally occurring element that is essential in small amounts, but toxic at higher concentrations. EPA’s water quality criterion for selenium provides recommendations and information for establishing water quality standards under the Clean Water Act. Fish tissue monitoring is one of the methods that EPA recommends using to implement this selenium criterion, though the agency’s suggestions aren’t legal requirements.
DEQ is proposing to adopt these EPA recommendations as state law. The proposal would primarily impact OAC 252:740 (Implementation of Oklahoma’s Water Quality Standards). It would add fish tissue criteria as the preferred method for assessing waterbodies for selenium, with water column criteria being used if fish tissue is unavailable.
DEQ also proposed adding language to OAC 252:730 (Oklahoma’s Water Quality Standards) that would require the use of a fish tissue study for selenium site-specific criteria.
Current Oklahoma regulations for selenium provide water-based limits protecting fish and wildlife from chronic and acute effects. If adopted as planned according to EPA’s Revised 2021 Aquatic Life Ambient Water Quality Criteria, the rule would not only add whole body and muscle tissue limits for selenium but also precipitate lower water-based limits for rivers and lakes.
Other Proposed Changes to DEQ’s Water Quality Regulations
There are other proposed revisions to the OAC that will change requirements for water quality and infrastructure:
Oklahoma Pollutant Discharge Elimination System Standards (OAC 252:606)
Add/modify definitions to Subchapter 1, Introduction.
Add language to Subchapter 6, Point Source Discharges, to determine the reasonable potential for selenium.
Change language in Subchapter 6, Point Source Discharges, to bar monitoring frequency reductions for a parameter when the receiving water is impaired for that parameter.
Add an equation to Appendix L that details the method used to implement numerical criteria for toxic substances to protect the fish and wildlife propagation beneficial use.
Public Water Supply Construction Standards (OAC 252:626)
Change clarification section to remove “The maximum detention time of the rapid mix basin at design flow is 30 seconds” and replace this language with “Provide good mixing of the raw water with the chemicals applied and prevent deposition of solids in the mixing zone.”
Add language to Subchapter 9, Treatment, noting a rapid mix detention time of not more than 30 seconds.
Water Pollution Control Facility Construction (OAC 252:656)
Update Subchapter 13, Preliminary Treatment Standards, to allow other designs besides vortex-type grit chamber detention times that may be authorized with engineering justification.
Specify in Subchapter 16, Biological Treatment Standards, that 4-inch piping is a minimum requirement for return sludge piping.
Specify in Subchapter 27, Water Reuse, that Category 6 reclaimed water must be drawn from the effluent of the final treatment process unit, with the intake located within or immediately downstream of the disinfection unit where disinfection is provided.
What’s Next for Oklahoma’s Water Quality Standards?
The Oklahoma Water Quality Management Advisory Council will vote on these proposed changes on December 2 and provide recommendations. On January 21, 2026, the DEQ Environmental Quality Board will consider the council’s decision.